1. Field of the Invention
The present invention relates to data communications and particularly to cellular networks in data communications using time division multiple access (TDMA) systems.
The object of the invention is a method and an arrangement for separating signals which operate in the same channel. The solution according to the invention can be applied in time division multiple access systems used in cellular networks of data communications.
2. Description of the Prior Art
In the TDMA (Time Division Multiple Access) systems used in cellular networks of data communications the interferences caused by cells using the same frequency limit the capacity of the system. Such interferences are called Co-Channel Interferences (CCI). A fairly typical interference case in TDMA cellular networks is the presence of a single strong interfering factor. Although in the following we mainly concentrate on a case where there is only one strong interferer present, the solution according to the invention can also be applied in the case of several interferers.
The capacity of the TDMA cellular network system has been enhanced by increasing the geographical re-use of the frequencies and by simultaneously decreasing the cell sizes and reducing the transmission power. This solution, which is generally used, is not satisfactory because its infrastructure becomes considerably inefficient and expensive.
A conventional approach to the cochannel interferences is to treat them as random white Gaussian noise in receivers. This approach is only satisfactory if the CCI interferences are close to the conventional noise level; however, this is generally not the case in cellular systems which are typically fairly interference-free. The CCI interferences are fairly deterministic in nature implying that we should be able to remove at least part of their negative influence.
Instead of attenuating the interference channel, the latest techniques show how to jointly estimate both the correct signal and the interfering signal. This technique is described, for instance, in the article by K. Giridhar et al: "Joint Estimation Algorithms for Cochannel Signal Demodulation", ICC 93, Conference Proceedings.
This is described briefly in the following with reference to FIG. 1. In FIG. 1, d.sub.1 (k) and d.sub.2 (k) are the symbols of the signal source of the first and, correspondingly, the second, i.e., the interfering channel. The pulse function g(t) has a raised-cosine pulse and its duration is 2T. After channels h.sub.1 (t) and h.sub.2 (t) the signals are combined and sampled in the T/2 sampling block. The separation of the signals is based either on the JMLSE (Joint Maximum Likelihood Sequence Estimation) or the JMAPSD (Joint Maximum A Posteriori Symbol Detection).
Problem to be solved:
Cochannel interferences often cause problems in spite of the above-described equalizer solutions according to a known technique and the parameters, i.e., impulse responses of the channel considerably correlate with each other. The situation is especially problematic with the above-described solutions when the power of the interference signal is almost as high as the power of the effective signal, or when their respective powers correlate with each other.